1. Technical Field
This invention relates to semiconductor manufacture, and more particularly, to an apparatus and method for chemical mechanical polishing (CMP) and in-situ endpoint detection during the CMP process.
2. Background Art
In the process of fabricating modern semiconductor integrated circuits (ICs), it is necessary to form various material layers and structures over previously formed layers and structures. However, the prior formations often leave the top surface topography of an in-process wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches, and/or other surface irregularities. These irregularities cause problems when forming the next layer. For example, when printing a photolithographic pattern having small geometries over previously formed layers, a very shallow depth of focus is required. Accordingly, it becomes essential to have a flat and planar surface, otherwise, some parts of the pattern will be in focus and other parts will not. In fact, surface variations on the order of less than 1000 xc3x85 over a 25xc3x9725 mm die would be preferable. In addition, if the aforementioned irregularities are not leveled at each major processing step, the surface topography of the wafer can become even more irregular, causing further problems as the layers stack up during further processing. Depending on the die type and the size of the geometries involved, the aforementioned surface irregularities can lead to poor yield and device performance. Consequently, it is desirable to effect some type of planarization, or leveling, of the IC structures. In fact, most high density IC fabrication techniques make use of some method to form a planarized wafer surface at critical points in the manufacturing process.
One method for achieving the aforementioned semiconductor wafer planarization or topography removal is the chemical mechanical polishing (CMP) process. In general, the chemical mechanical polishing (CMP) process involves holding and/or rotating the wafer against a rotating polishing platen under a controlled pressure. As shown in FIG. 1, a typical CMP apparatus 10 includes a polishing head 12 for holding the semiconductor wafer 14 against the polishing platen 16. The aforementioned polishing platen 16 is typically covered with a pad 18. This pad 18 typically has a backing layer 20 which interfaces with the surface of the platen and a covering layer 22 which is used in conjunction with a chemical polishing slurry to polish the wafer 14. Although some pads 18 have only the covering layer 22, and no backing layer. The covering layer 22 is usually either an open cell foamed polyurethane (e.g. Rodel IC1000), or a sheet of polyurethane with a grooved surface (e.g. Rodel EX2000). The pad material is wetted with the aforementioned chemical polishing slurry containing both an abrasive and chemicals. One typical chemical slurry includes KOH (Potassium Hydroxide) and fumed-silica particles. The platen is usually rotated about its central axis 24. In addition, the polishing head is usually rotated about its central axis 26, and translated across the surface of the platen 16 via a translation arm 28. Although just one polishing head is shown in FIG. 1, CMP devices typically have more than one of these heads spaced circumferentially around the polishing platen.
A particular problem encountered during a CMP process is in the determination that a part has been planarized to a desired flatness or relative thickness. In general, there is a need to detect when the desired surface characteristics or planar condition has been reached. This has been accomplished in a variety of ways. Early on, it was not possible to monitor the characteristics of the wafer during the CMP process. Typically, the wafer was removed from the CMP apparatus and examined elsewhere. If the wafer did not meet the desired specifications, it had to be reloaded into the CMP apparatus and reprocessed. This was a time consuming and labor-intensive procedure. Alternately, the examination might have revealed that an excess amount of material had been removed, rendering the part unusable. There was, therefore, a need in the art for a device which could detect when the desired surface characteristics or thickness had been achieved, in-situ, during the CMP process.
Several devices and methods have been developed for the in-situ detection of endpoints during the CMP process. For instance, devices and methods that are associated with the use of ultrasonic sound waves, and with the detection of changes in mechanical resistance, electrical impedance, or wafer surface temperature, have been employed. These devices and methods rely on determining the thickness of the wafer or a layer thereof, and establishing a process endpoint, by monitoring the change in thickness. In the case where the surface layer of the wafer is being thinned, the change in thickness is used to determine when the surface layer has the desired depth. And, in the case of planarizing a patterned wafer with an irregular surface, the endpoint is determined by monitoring the change in thickness and knowing the approximate depth of the surface irregularities. When the change in thickness equals the depth of the irregularities, the CMP process is terminated. Although these devices and methods work reasonably well for the applications for which they were intended, there is still a need for systems which provide a more accurate determination of the endpoint.
The present invention is directed to a novel apparatus and method for endpoint detection which can provide this improved accuracy. The apparatus and method of the present invention employ interferometric techniques for the in-situ determination of the thickness of material removed or planarity of a wafer surface, during the CMP process.
Specifically, the foregoing objective is attained by an apparatus and method of chemical mechanical polishing (CMP) employing a rotatable polishing platen with an overlying polishing pad, a rotatable polishing head for holding the wafer against the polishing pad, and an endpoint detector. The polishing pad has a backing layer which interfaces with the platen and a covering layer which is wetted with a chemical slurry and interfaces with the wafer. The wafer is constructed of a semiconductor substrate underlying an oxide layer. And, the endpoint detector includes a laser interferometer capable of generating a laser beam directed towards the wafer and detecting light reflected therefrom, and a window disposed adjacent to a hole formed through the platen. This window provides a pathway for the laser beam to impinge on the wafer, at least during the time that the wafer overlies the window.
The window can take several forms. Among these are an insert mounted within the platen hole.+This insert is made of a material which is highly transmissive to the laser beam, such as quartz. In this configuration of the window, an upper surface of the insert protrudes above a surface of the platen and extends away from the platen a distance such that a gap is formed between the upper surface of the insert and the wafer, whenever the wafer is held against the pad. This gap is preferably made as small as possible, but without allowing the insert to touch the wafer. Alternately, the window can take the form of a portion of the polishing pad from which the adjacent backing layer has been removed. This is possible because the polyurethane covering layer is at least partially transmissive to the laser beam. Finally, the window can take the form of a plug formed in the covering layer of the pad and having no backing layer. This plug is preferably made of a polyurethane material which is highly transmissive to the laser beam.
In one embodiment of the present invention, the hole through the platen, and the window, are circular in shape. In another, the hole and window are arc-shaped. The arc-shaped window has a radius with an origin coincident to the center of rotation of the platen. Some embodiments of the invention also have a laser beam whose beam diameter that at its point of impingement on the wafer is significantly greater than the smallest diameter possible for the wavelength employed.
The aforementioned CMP apparatus can also include a position sensor for sensing when the window is adjacent the wafer. This ensures that the laser beam generated by the laser interferometer can pass unblocked through the window and impinge on the wafer. In a preferred embodiment of the invention, the sensor includes a flag attached along a portion of the periphery of the platen which extends radially outward therefrom. In addition, there is an optical interrupter-type sensor mounted to the chassis at the periphery of the platen. This sensor is capable of producing an optical beam which causes a signal to be generated for as long as the optical beam is interrupted by the flag. Thus, the flag is attached to the periphery of the platen in a position such that the optical beam is interrupted by the flag, whenever the laser beam can be made to pass unblocked through the window and impinge on the wafer.
Further, the laser interferometer includes a device for producing a detection signal whenever light reflected from the wafer is detected, and the position sensor includes an element for outputting a sensing signal whenever the window is adjacent the wafer. This allows a data acquisition device to sample the detection signal from the laser interferometer for the duration of the sensing signal from the position sensor. The data acquisition device then employs an element for outputting a data signal representing the sampled detection signal. This data acquisition device can also include an element for integrating the sampled detection signal from the laser interferometer over a predetermined period of time, such that the output is a data signal representing the integrated samples of the detection signal. In cases where the aforementioned predetermined sample period cannot be obtained during only one revolution of the platen, an alternate method of piece-wise data acquisition can be employed. Specifically, the data acquisition device can include elements for performing the method of sampling the detection signal output from the laser interferometer during each complete revolution of the platen for a sample time, integrating each sample of the detection signal over the sample time to produce an integrated value corresponding to each sample, and storing each integrated value. The data acquisition device then uses other elements for computing a cumulative sample time after each complete revolution of the platen (where the cumulative sample time is the summation of the sample times associated with each sample of the detection signal), comparing the cumulative sample time to a desired minimum sample time, and transferring the stored integrated values from the storing element to the element for calculating a summation thereof, whenever the cumulative sample time equals or exceeds the predetermined minimum sample time. Accordingly, the aforementioned output is a data signal representing a series of the integrated value summations from the summation element.
The data signal output by the data acquisition device is cyclical due to the interference between the portion of the laser beam reflected from the surface of the oxide layer of the wafer and the portion reflected from the surface of the underlying wafer substrate, as the oxide layer is thinned during the CMP process. Accordingly, the endpoint in a CMP process to thin the oxide layer of a blank oxide wafer can be determined using additional apparatus elements for counting a number of cycles exhibited by the data signal, computing a thickness of material removed during one cycle of the output signal from the wavelength of the laser beam and the index of refraction of the oxide layer of the wafer, comparing a desired thickness of material to be removed from the oxide layer to a removed thickness comprising the product of the number of cycles exhibited by the data signal and the thickness of material removed during one cycle, and terminating the CMP whenever the removed thickness equals or exceeds the desired thickness of material to be removed. Alternately, instead of counting complete cycles, a portion of a cycle could be counted. The procedure is almost identical except that the thickness of material removed is determined for the portion of the cycle, rather than for an entire cycle.
An alternate way of determining the endpoint in a CMP processing of a blank oxide wafer uses apparatus elements which measure the time required for the data signal to complete either a prescribed number of cycles or a prescribed portion of one cycle, compute the thickness of material removed during the time measured, calculate a rate of removal by dividing the thickness of material removed by the time measured, ascertain a remaining removal thickness by subtracting the thickness of material removed from a desired thickness of material to be removed from the oxide layer, establish a remaining CMP time by dividing the remaining removal thickness by the rate of removal, and terminate the CMP process after the expiration of the remaining CMP time.
In addition, this remaining CMP time can be updated after each occurrence of the aforementioned number of cycles, or portions thereof, to compensate for any changes in the material removal rate. In this case the procedure is almost identical except that ascertaining the thickness of the material involves first summing all the thicknesses removed in earlier iteration and subtracting this cumulative thickness from the desired thickness to determine the remaining removal thickness figure.
However, when the wafer has an initially irregular surface topography and is to be planarized during the CMP process, the data signal is cyclical only after the wafer surface has become smooth. In this case an endpoint to the CMP process corresponding to a determination that the wafer has been planarized is obtained by employing addition apparatus elements for detecting a cyclic variation in the data signal, and terminating the CMP whenever the detecting element detects the cyclic variation. Preferably, the detecting element is capable of detecting a cyclical variation in the data signal within at most one cycle of the beginning of this variation.
In some circumstances, it is desirable to control the film thickness overlying a structure on a patterned wafer. This film thickness cannot always be achieved through the aforementioned planarization. However, this control can still be obtained by filtering the data signal to exclude all frequencies other than that associated with the particular structure, or group of similarly sized structures, over which a specific film thickness is desired. Essentially, once the signal has been filtered, any of the previously summarized ways of determining a CMP endpoint for a blank oxide wafer can be employed on the patterned wafer.